M1 macrophages produce pro-inflammatory cytokines such as IL-6, IL-12, TNF-α, MMP-2, and MMP-9, which promote the production of reactive oxygen species and nitric oxide to combat potential infection; in contrast, M2 macrophages can produce low levels of anti-inflammatory cytokines (IL-10, IL-14, and transforming growth factor-β) that promote the repair of damaged tissue [20,21,22]. The gene discussed is MMP9; the disease is infection.